X-ray radiography can be performed by positioning an x-ray source on one side of an object (e.g., a patient or a portion thereof) and causing the x-ray source to emit x-rays through the object and toward an x-ray detector located on the other side of the object. As the x-rays pass through the object from the x-ray source, their energies are absorbed to varying degrees depending on the composition of the object, and x-rays arriving at the x-ray detector form a two-dimensional x-ray image (also known as a radiograph) based on the cumulative absorption through the object.
Intraoral radiography is a technique in which an imaging sensor is placed inside the mouth of a patient and an x-ray source outside the mouth is used to irradiate the sensor with x-rays. The x-ray attenuation of hard tissues in the mouth results in a clinical image being formed on the sensor. Intraoral x-ray images provide a high level of detail of the tooth, bone, and supporting tissues. They also allow dentists to find cavities, examine tooth roots, evaluate the condition of the bony area around the tooth, determine if periodontal disease is present or a concern, and monitor the status of developing teeth, among other things.
First, increasing the applied x-ray dose typically improves the number of x-ray photons contributing to the image. Given that x-ray images are typically dominated by Poisson noise, the signal-to-noise ratio (SNR) improves as additional x-ray dose is applied. A minimum x-ray dose is therefore typically required to successfully visualize a given feature of clinical interest. Beyond that dosage, increasing dosage does not necessarily result in significant additional clinical utility.
Conventional x-ray imaging, discussed above, produces a two-dimensional image. Tomosynthesis however provides three-dimensional information about a patient in the form of tomographic image slices reconstructed from x-ray images of the patient taken from multiple perspectives within a scan angle smaller than that of computed tomography (CT) or cone-beam computed tomography (CBCT) (e.g., ±20°, compared with at least 180° in CBCT). However, tomosynthesis is a relatively undeveloped field in dentistry.
In both traditional x-ray imaging and tomosynthesis, an intraoral sensor/detector may be placed in a patient's mouth. For diagnostic images that include multiple teeth or for diagnostic tasks requiring entirely capturing a single tooth in an image, the size of a typical intraoral sensor can be prohibitive. A human's intraoral cavity has limited space, and thus the physical size of the intraoral sensor is also limited. In addition, patients may have certain conditions (e.g., dental tori) that restrict the use of intraoral sensors due to patient discomfort. There have been several approaches to increasing the field-of-view of the intraoral sensor. Some approaches focus on physical changes to the intraoral sensor. For example, one approach has been to use intraoral sensors with cut-off corners thereby making them easier to fit into the mouth. While this may allow for a larger intraoral sensor, this approach only marginally increases the field of view. Another approach has been to develop flexible intraoral sensors. This approach, however, requires significant changes in manufacturing parameters and does not appreciably increase the field of view. Another approach has been to capture and combine a series of images taken with parallel illumination. However, the typical system geometries for intraoral imaging result in significant stitching artifacts with this approach, causing misalignment between subvolumes to be combined. Other approaches rely on reconstruction methods to increase the reconstructed volume. These approaches are for external (i.e., non-intraoral) tomographic imaging systems where sample to be imaged is rotated, something which is impossible to achieve intraorally.
Therefore, it would be desirable to have a device, method and computer program products that could increase the effective size of a sensor to allow for viewing more teeth than can be seen with a standard sensor or, conversely, obtaining a standard size intraoral image on a patient who is unable to tolerate a sensor of standard size.
Further, intraoral x-ray imaging is a known and commonly used technology that is used to screen for caries and other dental pathologies. Instead of acquiring a single image using a stationary x-ray source, a series of images are taken while varying the source position in a known way. That series of images may be used to construct an estimate of the x-ray attenuation coefficient in the sampled volume. Intraoral radiography is a known and familiar technology which clinicians have considerable experience in evaluating. Therefore, providing both an intraoral radiograph and a dental tomosynthesis scan to a clinician will improve diagnostic capability. This has been solved in the past by presenting a center projection of a tomosynthesis scan as a radiograph. However, the center projection is not equivalent to a high dose radiograph because each projection of a tomosynthesis scan is typically taken at low dose. Another solution has been attempted in the past by moving the scanned x-ray source to the center of the scan position and then acquiring a high dose intraoral radiograph. However, this solution also increases the delivered dose to the patient by necessitating an additional high dose image which is not desirable.
In the case of breast tomosynthesis, a solution to generating a single two-dimensional image with significantly higher signal-to-noise ratio has involved reconstructing the tomosynthesis scan and then reprojecting the resulting volume to obtain a low noise mammogram by summing slices of the volume. Herein, non-iterative reconstruction methods are used wherein projections are acquired and filtered using a generalized Fourier filter. The filtered projection images are then backprojected to create a reconstructed volume. The reconstructed volume may then be reprojected to obtain a 2D image by summing slices that make up reconstructed volume. Filtered backprojection is a common non-iterative reconstruction technique. Each image is filtered and backprojected through a volume. The filter is typically chosen so that backprojections through the volume match the original projections. Artifacts may be minimized by smoothly extrapolating the input images so that the extrapolated images cover the full extent of the reconstructed volume. Unfortunately, this solution generates image artifacts when high contrast features move off of the field of view because the projection extensions are attempting to extrapolate large, high-frequency features, which is difficult to achieve.
Another problem with this solution is that the images taken during the scan contain information from different, overlapping volumes. The contrast variations are however relatively small. This method has therefore not been previously applied to hard tissues, such as dental anatomy which has high contrast variations or while using an intraoral scan. Dental tissues, unlike most breast tissues, particularly in patients with significant dental work containing metal, contain regions of extreme contrast variation. This contrast variation results in large truncation artifacts in reconstructed data which manifest in a reprojected radiograph. Truncation artifacts appear as multiple fine parallel lines immediately adjacent to high-contrast interfaces or as dark shading adjacent to high attenuation regions. They occur as a result of variations in the number of projections contributing to different regions in the reconstructed data. In addition, unlike breast tomosynthesis, the system geometry in dental tomosynthesis is not accurately known and the patient does not remain effectively static during scanning. In order to enable clinical usage at a range of positions in the mouth, an x-ray source may be mounted on a flexible arm. This arm is placed and aligned manually, with the expectation of significant variation in source placement depending on the user. In addition, the arm flexes and vibrates during the scan owing to the translation of the x-ray source. Second, breast tomosynthesis is also typically conducted with significantly larger pixel sizes and with the breast tissue fixed in place using an adjustable paddle. As a result, patient motion creates much more significant artifacts for intraoral tomosynthesis than for breast tomosynthesis. As such, it is necessary to measure the system geometry and patient position accurately. The simplest method involves the use of marker particles visible in the projections that can be used to determine the system geometry. Unfortunately, the use of marker particles generates artifacts in the reprojected radiograph.
Therefore, it would be desirable to have a device which allows for the provision of a low noise intraoral radiograph with features comparable to a standard radiograph given a low-dose tomosynthesis scan.